Solar cell and process for producing the same

ABSTRACT

The present invention provides a method of manufacturing a solar cell, comprising forming a buffer layer comprising a group-III nitride semiconductor on a substrate using a sputtering method, and forming a group-III nitride semiconductor layer and electrodes on the buffer layer. The group-III nitride semiconductor layer is formed on the buffer layer by at least one selected from the group consisting of the sputtering method, a MOCVD method, an MBE method, a CBE method, and an MLE method, and the electrodes are formed on the group-III nitride semiconductor layer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. application Ser. No. 12/033,603filed on Feb. 19, 2008, which claims benefit of U.S. ProvisionalApplication No. 60/905,820, filed on Mar. 9, 2007, and claims priorityof Japanese Patent Application No. 2007-38161 filed on Feb. 19, 2007,the contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a solar cell and a method ofmanufacturing the same, and more particularly, to a solar cell having alaminate structure of group-III nitride semiconductors (hereinafter,referred to as group-III nitride compound semiconductors) capable ofachieving both mass productivity and excellent characteristics and to amethod of manufacturing the same.

2. Description of the Related Art

It has been about thirty years since a technique for a solarphotovoltaic system was developed, and energy consumption has rapidlyincreased all over the world. Meanwhile, most the countries ratified theKyoto protocol in order to reduce greenhouse gases that cause climatechange, and thus many countries have turned their attention to naturalenergy, particularly, solar photovoltaic power generation.

In particular, the solar photovoltaic market has been rapidly expandingsince each country subsidized the development of the solar photovoltaicsystem. Due to an increase in the demand for silicon caused by thedevelopment of the semiconductor industry, a shortage in the supply of asilicon raw material has occurred in the solar photovoltaic market.

That is, with a rapid increase in demand for a polycrystalline siliconsolar cell, which is the main product, a failure has occurred in theproduction plan of a crystalline raw material, and it is expected that ashortage in supply of the silicon raw material will occur in the nearfuture. For this reason, the demand for reducing the thickness ofsilicon substrates has increased.

As a thin film solar cell, a solar photovoltaic system using, forexample, an amorphous silicon thin film or a microcrystalline siliconthin film, has been known. In terms of development, a solar photovoltaicsystem using a compound semiconductor thin film formed of, for example,a CIS-based thin film having Cu, In, or Se as a base material, or aCIGS-based thin film containing Ga, has been paid attention.

Further, a solar photovoltaic system using a dye sensitization-typeelement containing a Ru-based pigment or an organic thin film element,which enables a flexible solar photovoltaic element, has been developed.

In order to expand the solar photovoltaic market, it is necessary toimprove photoelectric conversion efficiency and reduce manufacturingcosts. An InGaP/GaAs multi junction solar cell using a GaAs tunneljunction layer, which has been used as a power source for, for example,a satellite, has photoelectric conversion efficiency considerably higherthan a silicon-based solar cell. However, has manufacturing costs areastronomically higher than the silicon-based solar cell. Therefore, itis difficult to use the In GaP/GaAs-based multi junction solar cell forgeneral household purposes.

As examples of the solar cell using the compound semiconductor, thefollowing have been proposed: a method of depositing a semiconductormulti-layer structure required for a multi-junction solar cell using anMOCVD method, thereby manufacturing a solar cell (for example, PatentDocument 1 (Japanese Unexamined Patent Application Publication No.2001-4445)); a 3-terminal multi-junction solar cell formed by depositingGe layers as a lower cell and GaAs layers as an upper cell using theMOCVD method (for example, Patent Document 2 (Japanese Unexamined PatentApplication Publication No. 2002-368238)); and a concentratingphotovoltaic apparatus having a concentrator in an InGaP/InGaAs/Ge basedmulti junction solar cell (for example, Patent Document 3 (JapaneseUnexamined Patent Application Publication No. 2006-313810)).

As a multi-junction tandem solar cell using a GaN-based material, thefollowing has been proposed: a multi-junction tandem solar cellincluding a plurality of p-n junction layers that are formed bydepositing In_((1-x))Ga_((x))N (Eg is in a range of about 0.7 eV to 3.4eV)-based thin films using an MBE method. However, details of amanufacturing method and performances thereof are not disclosed (forexample, Patent Document 4 (U.S. Published Application No,2004-0118451)).

The solar photovoltaic systems using the compound semiconductorsaccording to the related art have problems in that methods of depositingsolar cell element thin films are complicated, there is no disclosure ofa method of increasing the size of an element substrate, powerconsumption is considerably higher than other energy sources, and themanufacturing costs are higher than those of the polycrystalline siliconsolar cell according to the related art.

Meanwhile, as a method of growing crystals of a group-III nitridecompound semiconductor, the following have been proposed: an MOCVDmethod of reacting ammonia with an organic metal, such as organicgallium or an organic indium compound, at a high temperature during themanufacture of a light emitting element; an MBE method; and a sputteringmethod (for example, Patent Document 5 (Japanese Unexamined PatentApplication Publication No. 60-39819), Non-Patent Document 1 (UshikuYukiko et al., Deposition of GaN Thin Film Using RF Magnetron SputteringMethod (I), Second 21st Century Joint Symposium-Science and Technology,and Human Being-(2003, Tokyo)) and Non-Patent Document 2 (Asami Akinoriet al., Growth of GaN Single Crystal doped with Si or Mg by UHVSputtering Method, 66th Meeting of the Japanese Society of AppliedPhysics (The University of Tokushima, autumn 2005)). However, the methodof growing crystal using the MBE method or the sputtering method has notbeen industrially used for a light emitting element of the compoundsemiconductor or the manufacture of a solar cell element.

SUMMARY OF THE INVENTION

The present invention has been made in an effort to solve the aboveproblems, and an object of the present invention is to provide a solarcell that is useful for industry and a method of manufacturing the same.

The present invention has the following characteristics.

According to a first aspect of the present invention, a solar cellcomprises: a substrate; a buffer layer that is formed on the substrateand is composed of a group-III nitride semiconductor; and group-IIInitride semiconductor layers (p-type layer/n-type layer) having a p-njunction therein and is formed on the buffer layer. At least oneselected from the group consisting of the buffer layer and the group-IIInitride semiconductor layer having the p-n junction therein has acompound semiconductor layer formed by a sputtering method.

A second aspect of the solar cell according to the first aspect, aplurality of the group-III nitride semiconductor layers (p-typelayer/n-type layer) having the p-n junction therein may be comprised inthe solar cell element.

A third aspect of the solar cell according to the first aspect or thesecond aspect, the group-III nitride semiconductor layers (p-typelayer/n-type layer) having the p-n junction therein, which is formed onthe buffer layer, may be formed of In_(x)Ga_((1-x))N (0≦x<1).

A fourth aspect of the solar cell according to any one of the firstaspect to the third aspect, the substrate may be formed of at least oneselected from the group consisting of quartz, glass, sapphire, SiC,silicon, zinc oxide, magnesium oxide, manganese oxide, zirconium oxide,manganese zinc iron oxide, magnesium aluminum oxide, zirconium boride,gallium oxide, indium oxide, lithium gallium oxide, lithium aluminumoxide, neodymium gallium oxide, lanthanum strontium aluminum tantalumoxide, strontium titanium oxide and titanium oxide.

A fifth aspect of the solar cell according to any one of the firstaspect to the fourth aspect, an n-type electrode and a p-type electrodemay be formed on at least a portion of or the entire surface of thegroup-III nitride semiconductor layer having the p-n junction therein.

A sixth aspect of the solar cell according to any one of the firstaspect to the fifth aspect, the buffer layer may be formed of AlN orGaN.

A seventh aspect of the present invention, there is provided a method ofmanufacturing a solar cell. The method comprises: forming a buffer layercomposed of a group-III nitride semiconductor on a substrate using asputtering method; and forming a group-III nitride semiconductor layerand electrodes, on the buffer layer. The group-III nitride semiconductorlayer is formed on the buffer layer by at least one selected from thegroup consisting of the sputtering method, a MOCVD method, an MBEmethod, a CBE method, and an MLE method, and then the electrodes areformed on the group-III nitride semiconductor layer.

An eighth aspect of the present invention according to the seventhaspect, at least one of group-III nitride semiconductor layers (p-typelayer/n-type layer) having a p-n junction therein that are included inthe group-III nitride semiconductor layer formed on the buffer layer maybe formed by the sputtering method.

A ninth aspect of the present invention according to the seventh aspector the eighth aspect, the sputtering method may comprise reactingnitrogen in a plasma, radical, or atomic state with a group-III element,thereby forming the layer.

A tenth aspect of the present invention according to any one of theseventh aspect to the ninth aspect may further comprise a first step ofalternately repeating a process of supplying only dopant element and aprocess of simultaneously supplying a compound including group-IIIelement and a nitrogen raw material.

An eleventh aspect of the present invention according to the tenthaspect may further include: a second step of performing a heat treatmenton the layer formed by the first step.

A twelfth aspect of the present invention according to the eleventhaspect, a heat treatment temperature may be in a range of about 300° C.to about 1200° C.

A thirteenth aspect of the present invention according to the eleventhaspect or the twelfth aspect, the heat treatment may be performed in anatmosphere that does not contain a gas of a compound comprising hydrogenatom and hydrogen gas.

A fourteenth aspect of the present invention according to any oneselected from the group consisting of the tenth aspect to the thirteenthaspect, the dopant element may be at least one of Si, Ge, and Sn.

A fifteenth aspect of the present invention according to any one of thetenth aspect to the thirteenth aspect, the dopant element may be atleast one of Mg and Zn.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view schematically illustrating an exampleof the structure of a solar cell element including a GaN junction layer,manufactured according to Example 1 in the present invention;

FIG. 2 is a plan view illustrating an example of the electrode structureof the solar cell element manufacturing according to Example 1 in thepresent invention;

FIG. 3 is a cross-sectional view schematically illustrating an exampleof the structure of a solar cell element including an In_(X1)Ga_(1-X1)Njunction layer, manufactured according to Example 2 in the presentinvention; and

FIG. 4 is a cross-sectional view schematically illustrating an exampleof the structure of a solar cell element having two junctions of a GaNjunction layer and an In_(X2)Ga_(1-X2)N junction layer according to thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to a solar cell comprising at least a buffer layerthat is composed of a group-III nitride semiconductor and a group-IIInitride semiconductor layers (p-type layer/n-type layer) having a p-njunction therein, formed on a substrate, wherein at least one selectedfrom the group consisting of the buffer layer and the group-III nitridesemiconductor layers having the p-n junction therein has a compoundsemiconductor layer formed by a sputtering method and to a method ofmanufacturing the same.

Further, in the present invention, both an underlying layer that iscomposed of a group-III nitride semiconductor and is formed on thebuffer layer and each of the group-III nitride semiconductor layers (ptype layer/n-type layer) having a p-n junction therein are formed by thesputtering method.

Furthermore, in the present invention, a solar cell comprises thegroup-III nitride semiconductor layers having a p-n junction therein andmay include at least a semiconductor multi-layer film in which an n-typelayer and a p-type layer are preferably laminated in this order in thedirection in which sunlight is incident. However, the laminateddirection is not limited to the incident direction of sunlight. Inaddition, a multi junction solar cell having a tandem structureincluding a plurality of semiconductor layers, each having a p-njunction therein, may be formed on the same substrate.

In the solar cell according to the present invention, the group-IIInitride semiconductor layer having a p-n junction therein, which isformed on the buffer layer, is preferably formed of In_(x)Ga_((1-x))N(0≦x<1). Here, x indicates an arbitrary atom ratio, and is representedby, for example, X1 or X2, which will be described below. This issimilarly applied to the following description hereinafter.

The buffer layer is preferably formed of GaN or AlN, but the inventionis not limited thereto. The thickness of the buffer layer is generallyin a range of about 1 nm to about 1000 nm, preferably, about 3 nm toabout 400 nm, more preferably, about 5 nm to about 200 nm.

After the buffer layer is formed, an underlying layer may be formedbetween the buffer layer and the p-n junction layer. The underlyinglayer is formed of a group-III nitride semiconductor, preferably,In_(x)Ga Ga_((1-x))N (0≦x<1), more preferably, GaN layer.

Further, in the present invention, when the buffer layer is formed by asputtering method, the buffer layer has high uniformity. As a result,when the underlying layer is subsequently formed on the buffer layer, itis possible to reduce the threading dislocation in the underlying layer.In particular, the buffer layer formed by the sputtering method has athickness smaller than that of the buffer layer formed by an MOCVDmethod, also has in-plane uniformity higher than that of the bufferlayer formed by the MOCVD method. For example, it is possible to obtainan underlying layer having the following characteristics: the rockingcurve half-width of a (0002) plane is equal to or less than about 100arcsec; and the rocking curve half-width of a (10-10) plane is equal toor less than about 300 arcsec.

Since the underlying layer having excellent characteristics is formed, asolar cell having a p-n junction which is formed on the underlying layercan have excellent characteristics on the property of the solar cell. Inaddition, even when the underlying layer is not formed, it is possibleto produce a solar cell with excellent characteristics due to forming agroup-III nitride semiconductor layers (p-type layer/n-type layer)having a p-n junction therein.

Furthermore, a substrate may be put into a chamber of a sputteringapparatus, and may be carried out by pre-treatment such as sputteringbefore a buffer layer is formed. Specifically, a pre-treatment ofcleaning the surface of the substrate by exposing the substrate to Ar orN₂ plasma in the chamber may be performed. It is possible to remove anorganic material or an oxide adhered to the surface of the substrate byexposing the substrate to the plasma, such as Ar gas or N₂ gas. In thiscase, when a voltage is applied between the substrate and the chamber,not to a target, plasma particles effectively act on the substrate.

After the pre-treatment is performed on the substrate, the buffer layershown in FIG. 1 is formed on the substrate by the sputtering method.

In general, the sputtering method can maintain the substrate at a lowtemperature. Therefore, even when a substrate formed of a material thatis decomposed at a high temperature is used, the sputtering method makesit possible to form a layer on the substrate 11 without damaging thesubstrate.

Furthermore, in the present invention, the crystallinity of the bufferlayer is not particularly limited. The buffer layer may be polycrystal(for example, columnar crystal) or single crystal. It is preferable thatthe buffer layer be single crystal.

The underlying layer, if necessary, may be doped with n-type impuritiesor it may be undoped (<1×10¹⁷/cm³). Preferably, the underlying layer isundoped in order to maintain good crystallinity.

Further, the underlying layer may be an electro conducting layer bydoping with dopant, thereby forming electrodes on and underneath a p-njunction element.

The thickness of the underlying layer is generally in a range of about0.01 μm to about 30 μm, preferably, about 0.05 μm to about 20 μm, morepreferably, about 0.1 μm to about 10 μm. However, the invention is notlimited thereto.

Group-III nitride semiconductor layers having a plurality of p-njunctions therein may be connected to each other by a tunnel junction orohmic electrodes.

In an embodiment of the present invention, the substrate (a differentkind of substrate) may be different from a substrate composed of agroup-III nitride semiconductor or a material that is crystal-grown on asubstrate. For example, as different kind of substrate, one selectedfrom the group consisting of quartz, glass, sapphire, SiC, silicon, zincoxide, magnesium oxide, manganese oxide, zirconium oxide, manganese zinciron oxide, magnesium aluminum oxide, zirconium boride, gallium oxide,indium oxide, lithium gallium oxide, lithium aluminum oxide, neodymiumgallium oxide, lanthanum strontium aluminum tantalum oxide, strontiumtitanium oxide, and titanium oxide may be preferably used. Morepreferably, at least one selected from the group consisting of quartz,glass, sapphire, SiC, and silicon may be used. More preferably, thesubstrate may be used of quartz, glass, or sapphire.

Hereinafter, exemplary embodiments of the present invention will bedescribed with reference to the accompanying drawings. The drawings aresimply illustrative, but the scale of each layer or member is adjustedin order to have a recognizable size in the drawings.

Next, a solar cell having the above-mentioned structure will bedescribed. In FIGS. 1, 3, and 4, the incident direction of sunlight isnot limited to that shown in the drawings, but sunlight may be incidentin the opposite direction of that shown in the drawings.

FIG. 1 is a longitudinal cross-sectional view illustrating an example ofthe structure of a solar cell element that has a p-n junction thereinand is formed of GaN. A p-type GaN layer 50 and an n-type GaN layer 51that have a p-n junction therebetween are laminated on a quartzsubstrate 1 with an MN buffer layer 2 and an undoped GaN underlyinglayer 3 with interposed therebetween, and an ITO (indium tin oxide) ofan ohmic electrode 60, serving as a current collecting electrode, isformed on the semiconductor layer having a p-n junction therein. Thesurface of the electrode may be covered with a protective film 61, ifnecessary. Any layer may be used as the protective film 61 as long as itcan protect the ohmic electrode. A known antireflection film (forexample, a MgF₂/ZnS₂ film) is formed on the surface of the laminate onwhich sunlight is incident.

FIG. 3 is a longitudinal cross-sectional view illustrating an example ofthe structure of a solar cell element including p-type and n-type InGaNlayers 70 and 71 that are formed of In_(X1)Ga_((1-X1))N and have a p-njunction therebetween, instead of the p-type and n-type GaN layers 50and 51 having a p-n junction therebetween shown in FIG. 1.

In this embodiment of the present invention, a plurality of compoundsemiconductor layers each having a p-n junction may be formed on thesubstrate. In this case, a tunnel junction layer having a high tunnelpeak current density is formed between the junction layers. FIG. 4 is alongitudinal cross-sectional view illustrating an example of thestructure of a solar cell element including two compound semiconductorlayers, each having a p-n junction therein, formed on a substrate.

In a group III-V compound semiconductor layer, generally, group-Vmolecules are evaporated from the surface of the layer at a temperatureof about 400° C. or more. Therefore, it is possible to obtain a tunnelpeak current density of 50 mA/cm² or more by performing heat treatmenton the tunnel junction layer formed of In_(x)Ga_((1-x))N (0≦x<1) at arelatively high temperature of about 400° C. to about 800° C. in a shortamount of time (for example, several seconds). However, the thermaltreatment may be performed for ten minutes or more at a relatively lowtemperature of about 300° C. to about 400° C. Since the tunnel junctionlayer also absorbs light and causes a light absorption loss, the tunneljunction layer may have a sufficiently small thickness to obtain desiredtunnel junction characteristics, preferably, a thickness of 10 nm orless.

In FIGS. 1 and 3, an ohmic contact electrode (ohmic electrode) 60,serving as a rear electrode, formed of, for example, ITO is formed onone surface of the p-type GaN layer 51 or the p-type In_(X1)Ga_(1-X1)Nlayer 71, and an electrode formed of, for example, Au is formed on theohmic electrode 60. The surface of the compound semiconductor ispartially processed by a known photolithography method and a knownetching method (including dry etching) using the n-type GaN layer 50 orthe n-type In_(X1)Ga_(1-X1)N layer 70 as a contact layer, therebyforming a pad electrode. The pad electrode is formed of, for example,Cr, Ti, or Au.

For example, the AlN buffer layer shown in FIGS. 1 and 3 is formed onthe quartz substrate by a sputtering method. In this case, generally,the semiconductor layers other than the AlN buffer layer are epitaxiallygrown by a MOCVD method.

FIG. 4 is a longitudinal cross-sectional view illustrating an example ofthe structure of a solar cell element including the p-type and n-typeGaN layers (50 and 51) having a p-n junction therebetween and p-type andn-type In_(X2)Ga_(1-X2)N layers (80 and 81) having a p-n junctiontherebetween. The laminate structure of the compound semiconductorsshown in FIG. 4 can be manufactured by the same method as described withreference to FIGS. 1 and 3.

The n-type semiconductor according to this embodiment of the presentinvention is not particularly limited as long as it is doped with ann-type dopant showing the effect of the solar cell. For example, then-type semiconductor may preferably contain at least one kind of elementselected from the group consisting of Si, Ge, and Sn. The p-typesemiconductor is not particularly limited as long as it is doped with ap-type dopant showing the effect of the solar cell. For example, thep-type semiconductor may preferably contain at least one kind of elementselected from a group consisting of Mg and Zn.

In this embodiment of the present invention, a junction cell is notparticularly limited as long as it has the effect of the solar cell. Forexample, the junction cell may have a thickness of about 0.1 to about 3μm, each of the n-type In_(x)Ga_((1-x))N layer (0≦x<1) and the p-typeIn_(x)Ga_((1-x))N layer (0≦x<1) may have a thickness of about 20 nm toabout 1.5 μm, and the concentration of impurities may be in a range ofabout 4×10¹⁷ to 4×10¹⁸ cm⁻³.

Next, a method of forming the laminate structure of the group-IIInitride semiconductors forming the solar cell according to the presentinvention will be described.

In this embodiment, as described above, the AlN layer is formed on thesubstrate by the sputtering method, and the underlying layer is formedthereon by the sputtering method, the MOCVD method, or other methods.Then, the group-III nitride semiconductor layer having a p-n junctiontherein is formed on the underlying layer.

In this embodiment, when the sputtering method is used to form a dopinglayer of the group-III nitride semiconductor layer having the p-njunction, a layer containing a dopant atom and an undoped III nitridesemiconductor layer are alternately laminated as repeated, andsubsequently doping is performed on the laminate layers.

When a chemical vapor deposition method, such as the MOCVD method, isgenerally used, gas is mixed to perform doping. However, in theabove-mentioned method, it is possible to improve the productivity andreproducibility by the performance of the MOCVD method withoutperforming such as gas-mixing method.

In a method of physically forming a crystal film using the sputteringmethod, plasma, radical, or atomized nitrogen is supplied. It ispreferable that nitrogen not be supplied to a chamber during a processof supplying dopant atoms, in order to prevent the reaction between thedopant atoms and the nitrogen.

In the embodiment of the present invention, a method of alternatelyperforming a process of supplying only a dopant and a process of formingthe group-III nitride semiconductor using nitrogen is provided. In thelaminate structure formed by this method, layers containing only thedopant and the undoped group-III nitride semiconductor layers arealternately laminated. During the process of alternately laminating thelayers, some of the dopant atoms of the doping layer may be diffused tothe group-III nitride semiconductor layers. However, in this stage, thelayer containing only the dopant atoms is required.

Dopant comprising the layer may be p-type dopant or n-type dopant. Forexample, Mg or Zn has been used as the p-type dopant for the group-IIInitride semiconductor, and Si, Ge, or Sn has been used as the n-typedopant for the group-III nitride semiconductor. Among theabove-mentioned elements, preferably, Si is used as the n-type dopantand Mg is used as the p-type dopant since they have high dopingefficiency, high activation rate, and low reduction in crystallinity.

It is preferable that the group-III nitride semiconductor layer crystalforming the layer have an In_(x)Ga_((1-x))N (0≦x<1) structure.

When the doping layer fully covers the group-III nitride semiconductorlayer, an epitaxial relation is not established therebetween since theyhave different crystal lattice constants, which results in a reductionin crystallinity. For this reason, the doping layer is preferably formedto be scattered in island shapes on the surface of the group-III nitridesemiconductor layer without forming a complete layer. When the dopinglayer is formed in island shapes, the crystal of the group-III nitridesemiconductor is epitaxially grown in the exposed portions of the dopinglayer. In this way, it is possible to completely fill the exposedportions by epitaxial growth in the lateral direction.

It is preferable that the island-shaped portions of the island-shapeddoping layer may be arranged at an interval of about 2 nm to about 100nm in the width direction. If the gap between the island-shaped portionsis smaller than the above range, it is difficult for the group-IIIcompound semiconductor crystal to be epitaxially grown in the gap. Onthe other hand, if the gap between the island-shaped portions is largerthan the above range, dopants are not sufficiently diffused, whichcauses an increase in the driving voltage for an element. Morepreferably, the island-shaped portions are arranged at an interval ofabout 10 nm to about 50 nm in the width direction.

It is preferable that the ratio of the total area of the island-shapedportions of the island-shaped doping layer to the area of the entiresurface of the island-shaped doping layer be in a range of about notless than 0.001% to not more than 0.9%. If the total area is larger thanthe above range, it is difficult for the group-III compoundsemiconductor crystal to be epitaxially grown in the gap. On the otherhand, if the total area is smaller than the above range, dopants are notsufficiently diffused, which causes an increase in the driving voltagefor an element. More preferably, said ratio of the total area of theisland-shaped portions of the island-shaped doping layer is in a rangeof about 0.005 to 0.5 of the area of the entire surface of theisland-shaped doping layer.

It is preferable that the island-shaped portions of the doping layerhave a diameter (corresponding to the diameter of a circle) of about 0.5nm to about 100 nm. If the diameter of the island-shaped portion issmaller than the above range, dopants are not sufficiently diffused,which causes an increase in the driving voltage for an element. On theother hand, if the diameter of the island-shaped portion is larger thanthe above range, the crystallinity of the group-III compoundsemiconductor crystal is lowered. More preferably, the diameter of theisland-shaped portion is in a range of about 1 nm to about 10 nm.

The diameter of the island-shaped portions of the island-shaped dopinglayer and the gap between the island-shaped portions can be measured byobserving a sample with the cross-section thereof exposed using atransmission electron microscope. In addition, the ratio of the totalarea of the island-shaped portions to the area of the entire surface ofthe doping layer can be calculated by measuring the diameters of theisland-shaped portions and the gaps therebetween at, for example, 10measurement points selected by random sampling and calculating theaverage of the measured values.

Film deposition conditions may be considered in order to form the dopinglayer composed of the island-shaped portions. Since the doping layer isnot lattice-matched with the group-III nitride semiconductor layer, itis possible to form crystal islands under the controls that activemigration occurs.

For example, it is possible to set the temperature of the substrate toabout 600° C. or more, the pressure of the chamber to about 0.3 Pa orless during deposition, and the deposition rate to about 0.5 nm/sec orless. The thickness of the layer comprising only the dopant ispreferably in a range of about 0.1 nm to about 10 nm. If the thicknessis smaller than the above range, dopants are likely to be insufficientlydiffused. On the other hand, if the thickness is larger than the range,it is difficult to completely fill the exposed portions of the dopinglayer even when the group-III nitride semiconductor crystal isepitaxially grown in the lateral direction. More preferably, thethickness is in a range of about 0.5 nm to about 5 nm.

It is preferable that the thickness of the group-III nitridesemiconductor layer be in a range of about 1 nm to about 500 nm. If thethickness is larger than the range, dopants are likely to beinsufficiently diffused. On the other hand, if the thickness is smallerthan the range, it is difficult to completely fill up the exposedportions of the doping layer even when the group-III nitridesemiconductor crystal is epitaxially grown in the lateral direction.More preferably, the thickness is in a range of about 10 nm to about 100nm.

Further, it is preferable that the ratio of the thicknesses of the twolayers (the ratio of the thickness of the group-III nitridesemiconductor layer to the thickness of the doping layer) be in a rangeof about 10 to about 1000. If the ratio is smaller than the above range,an excessively large amount of dopant is doped, which causes thedeterioration of the crystallinity of the group-III nitridesemiconductor crystal. On the other hand, if the ratio is larger thanthe above range, dopants are not sufficiently diffused, and theresistance of the laminate structure increases. As a result, a drivingvoltage increases.

The number of repeating for laminates of the group-III nitridesemiconductor layer and the doping layer formed in the first process ispreferably in the range of 1 to about 200. Even when the repeatingnumber of laminates is larger than about 200, there is no improvement inthe performance of the element, but only the crystallinity of theelement is lowered.

For example, sputtering, PLD, PED, and CVD have been known as a methodof supplying a nitrogen raw material as plasma or radical. Among themethods, the sputtering method is preferable since it is easy to performand is suitable for mass production. A DC sputtering method may causethe charge-up of the surface of a target, and a deposition rate is morelikely to be unstable. Therefore, a pulse DC method or an RF sputteringmethod is preferable.

In the embodiment in the present invention, a known compound may be usedas a raw material for generating plasma or radical nitrogen.Particularly, it is preferable to use ammonia and nitrogen since theyare inexpensive and easy to handle. Ammonia has advantages in that ithas high degradation efficiency and can be deposited at a highdeposition rate. However, the ammonia has disadvantages in that it hashigh reactivity and toxicity, it is necessary to provide ammonia removalequipment or a gas detector, and members used for a reactor need to beformed of a material having high stability. On the other hand, whennitrogen is used as a raw material, the structure of an apparatus issimplified, but it is difficult to obtain a high reaction rate. A methodof decomposing nitrogen using an electric field or heat and introducingit into the apparatus can obtain a sufficiently high deposition rate touse, which is lower than that when ammonia is used. Therefore, nitrogenis the most preferable nitrogen material in consideration of themanufacturing costs and the balance of an apparatus.

When sputtering is used to form the layer, important parameters are thetemperature of the substrate, the internal pressure of a furnace, andthe partial pressure of nitrogen. The temperate of the substrate isgenerally in the range of room temperature to about 1200° C. If thetemperature is higher than about 1200° C., crystal decomposition occurs.Preferably, the temperature of the substrate is in a range of about 200°C. to about 900° C.

It is preferable that the internal pressure of the furnace be greaterthan or equal to about 0.3 Pa. If the internal pressure is lower thanthe value, the amount of nitrogen is reduced in the furnace, and thesputtered metal does not become a nitride, but is deposited to thelayer. The upper limit of the pressure is not particularly limited, butit should be understood that a sufficiently low pressure to generateplasma is required. It is preferable that, for the ratio of the flowrates of nitrogen to the flow rate of nitrogen and argon, nitrogen is ina range of about 20 percent by volume to about 100 percent by volume. Ifthe flow rate of nitrogen is lower than 20%, the sputtered metal isdeposited as metal. More particularly, the flow rate of nitrogen is in arange of about 50% to about 90%. If the flow rate is greater than orequal to about 90%, the amount of argon is small and the sputtering ratetends to be lowered.

It is preferable that the deposition rate be in a range of about 0.01nm/sec to about 10 nm/sec. If the deposition rate is higher than theabove range, a film is not crystallized, but becomes amorphous. If thedeposition rate is lower than the above range, a film is not formed, butthe film is grown in an island shape. Therefore, it is difficult tocover the surface of the substrate.

Meanwhile, since the doping layer is formed of a single component, areactive sputtering method is not preferable. Therefore, both an RFsputter and a DC sputter can be used as a sputtering apparatus. When theDC sputter is used, an electroconducting target may be used such thatthe target is not charged. For example, when a target with high purity,such as Si, is used, the target has an insulating property, and thecharge-up thereof should be considered. Therefore, it is preferable touse a target doped with B or P. However, in the DC sputter, it is awaste of time to reciprocate a wafer in the chamber when layers arealternately laminated as repeated. Therefore, it is preferable that thelayers be laminated in the same chamber as that in which the group-IIInitride semiconductor is made. Consequently, it is preferable to use theRF sputter.

It is preferable that the internal pressure of the furnace and thetemperature of the substrate in the process of forming the doping layerbe the same as those in the process of forming the group-III nitridesemiconductor, for the same reason as described above. The depositionrate is preferably lower than that in the process of forming thegroup-III nitride semiconductor. This is because it is easy toaccurately control the thickness of the layer. Preferably, thedeposition rate is in a range of about 0.001 nm/sec to about 1 nm/sec.

It is preferable that, after the doping layer and the group-III nitridesemiconductor layer are alternately laminated as repeated, to form alaminate structure, a heat treatment, which is a second process, beperformed on the laminate structure. The heat treatment makes itpossible for dopant to be diffused in the group-III nitridesemiconductor crystal. As a result, it is possible to perform uniformdoping state.

It is preferable that the heat treatment be performed at a temperatureof about 300° C. or more. The upper limit of the heat treatmenttemperature is not particularly limited, but the heat treatmenttemperature needs to be lower than the temperature at which matrixcrystal is decomposed. Generally, most of the group-III nitridesemiconductor crystal is decomposed at a temperature of about 1200° C.

The heat treatment time is not particularly limited, but it ispreferably in the range of about 30 seconds to about one hour. If theheat treatment time is shorter than about 30 seconds, it is difficult toobtain sufficient heat treatment effects. On the other hand, if the heattreatment time is longer than about one hour, there is no improvement inthe heat treatment effects, which results in a waste of time.

For the atmosphere during the heat treatment, particularly, when forminga p-type doping layer to manufacture a p-type laminate structure, it ispreferable not to use hydrogen gas and compound gas combined withhydrogen atoms in molecule. In particular, it is preferable not to useH₂ gas or NH₃ gas that is decomposed at a high temperature to generateH₂ gas.

In the laminate structure subjected to the heat treatment, the dopantmay form a crowd as the trace of the doping layer therein, depending onthe heat treatment time and temperature thereof. The mass of the dopantis diffused to be removed. For the concentration of the dopant, highlydoped layers and lightly doped layers may be alternately arranged. Thedopant may be uniformly diffused in to form an entirely homogeneousdoping layer.

Therefore, the laminate structure subjected to the heat treatment servesas a contact layer. When the p-type doping layer is used, the laminatestructure is used as a p-type contact layer. When the n-type dopinglayer is used, the laminate structure is used as an n-type contactlayer.

Electrodes for passing through-current are formed on the contact layer.The electrodes can be formed of any known material. For example, ann-type electrode may be formed, for example, Al, Ti, or Cr, etc. and ap-type electrode may be formed of, for example, Ni, Au, or Pt, etc. Theelectrodes may be formed of an electro conductive oxide, such as ITO,ZnO, AZO, or IZO.

EXAMPLES

Next, the invention will be described in detail with reference toExamples. However, the present invention is not limited to the followingExamples.

Example 1

In Example 1, a method of manufacturing a group-III nitridesemiconductor laminate structure forming a solar cell element will bedescribed.

FIG. 1 is a cross-sectional view illustrating the laminate structuremanufactured according to Example 1. The following was formed on thequartz substrate 1 available on the market: AlN buffer layer 2 with athickness of 30 nm, the undoped GaN layer 3 with a thickness of 0.1 μm,the n-type GaN layer 50 with a thickness of about 0.5 μm that was formedby alternately laminating Si-doped layer (having a thickness of 2 nm),which is composed of crystal islands, and undoped GaN layer (having athickness 100) nm as repeated for forty times and by performing a heattreatment on the laminate, and the p-type GaN layer 51 with a thicknessof 0.5 μm that was doped with Mg and manufactured by the same method asdescribed above. A method of manufacturing the n-type and p-type dopinglayers will be described below. The carrier concentration of the n-typeGaN layer 50 was about 2×10¹⁸ atoms/cm³, and the carrier concentrationof the p-type GaN layer was about 2×10¹⁸ atoms/cm³. These layers wereformed by an RF magnetron sputter.

A sputter in which the distance between a target and a cathode is 50 mmwas used for the manufacturing method. During the deposition of thelayers, the temperature of the substrate was 750° C., and the pressurewas 0.6 Pa. A substrate having a length of 30 cm (optical substrate),which is available on the market, was used as the quartz glasssubstrate.

In the manufactured laminate, the ohmic electrode layer 60 made of ITOwas formed on the surface of the p-type GaN layer 51 by a knownphotolithography technique, and a rear electrode, which was a laminateof a titanium film, an aluminum film, and a gold film formed in thisorder, was provided on the surface of the ohmic electrode layer. Insteadof the ITO, a separate crystalline IZO was used to form the ohmicelectrode. The crystalline IZO had high processability using the etchingproperty of an amorphous IZO, which was a precursor.

Then, a known etching process (for example, dry etching) was performedon the rear surface of the electrode of the laminate, thereby exposing aportion of the n-type GaN layer 50 for forming an n-type electrode, andthe n-type electrode, which is a laminate of four layers, that is, Ni,Al, Ti and Au layers, was manufactured in the exposed portion. In thisway, a solar cell element was manufactured.

In the process of forming the AlN buffer layer 2, a mixed gas of argonand nitrogen was introduced into the chamber, an electric field wasapplied to generate nitrogen plasma, and the nitrogen plasma was used asa nitrogen source, Meanwhile, argon ions in the plasma collided with anAl target, and metal atoms were emitted from the Al target and reactedwith nitrogen, thereby forming a film on the substrate.

In the process of forming the undoped GaN underlying layer 3, similar tothe process of forming the AlN layer, a mixed gas of argon and nitrogenwas introduced into the chamber, an electric field was applied togenerate nitrogen plasma, and the nitrogen plasma was used as a nitrogensource. Argon ions in the plasma collided with a Ga target, and Ga atomswere emitted from the Ga target and reacted with nitrogen, therebyforming a film on the substrate.

For a layer for forming the n-type doped layer, in the process ofalternately laminating the undoped GaN layer and the Si layer asrepeated, the undoped GaN layer was formed by the same method as thatused to form the undoped GaN underlying layer 3. The Si layer was formedas follows: only argon gas was introduced into the chamber; and argonions collided with a Si target, and Si atoms were emitted to the surfaceof the substrate.

The substrate manufactured in this way was taken out from the sputter,and put into an annealing furnace to be subjected to heat treatment. Aheat treatment temperature was 1100° C. and was maintained for 10minutes. The heat treatment was performed in only a nitrogen atmosphere.

Further, before and after the annealing process, the n-type GaN layer 50was observed by a transmission electron microscope in thecross-sectional direction. Before the annealing process, the laminatesas repeated for 40 times, each composed of a Si layer with a thicknessof 2 nm and an undoped GaN layer with a thickness of 100 nm, wereobserved in the laminated structure. The observation showed that the Silayer was not continuously formed, but had circular island portionsformed therein. The diameter of the circular island portion was about 1nm, and the gap between the circular island portions was about 50 nm.Therefore, the ratio of the total area of the doping layer to the entiresurface of the Si layer was about 0.02. In the n-type GaN layer 50 afterthe annealing process, a definite layer structure was not observed. Itwas considered that Si atoms of the doping layer were diffused and theGaN layer was uniformly doped.

The solar cell element substrate manufactured in this way was dividedinto square substrates with a length of 1 cm for clarity of description,and the electrodes of the square substrate were connected to a leadframe by gold lines, thereby manufacturing a solar cell element.

FIG. 2 is a plan view illustrating an example of the structure of theelectrode of the manufactured solar cell element.

Example 2

Next, another method of manufacturing the laminate structure shown inFIG. 1 will be described. An AlN buffer layer 2 with a thickness of 30nm was formed on the quartz substrate 1 using an RF magnetron sputter,and then taken out from the sputter. Then, the AlN buffer layer was putinto an annealing furnace, and heat treatment was performed thereon at atemperature of 1100° C. and under a nitrogen atmosphere for 10 minutes.Then, the treated substrate was put into an MOCVD furnace, and the samelaminate as that in Example 1, that is, a laminate including the p-typeGaN layer 51 doped with Mg was manufactured. In the MOCVD method, forexample, the temperature, the pressure, and the gas used were the sameas those in a known method. The carrier concentration of the n-type GaNlayer 50 was about 2×10¹⁸ atoms/cm³, and the carrier concentration ofthe p-type GaN layer 51 was about 2×10¹⁸ atoms/cm³.

A solar cell element substrate was manufactured by the same method asthat used in Example 1 using the manufactured laminate. The solar cellelement substrate manufactured in this way was divided into squaresubstrates with a length of 1 cm, and the electrodes of the squaresubstrate were connected to a lead frame by gold lines, therebymanufacturing a solar cell element.

Example 3

FIG. 3 is a cross-sectional view illustrating a laminate structureaccording to Example 3. An AlN buffer layer 2 with a thickness of 30 nmwas formed on the quartz substrate 1 using an RF magnetron sputter, andthen taken out from the sputter. Then, the AlN buffer layer was put intoan annealing furnace, and heat treatment was performed thereon at atemperature of 1100° C. and under a nitrogen atmosphere for 10 minutes.Then, the substrate was put into an MOCVD furnace, and the following aresequentially deposited on the substrate by a known MOCVD method (forexample, temperature, pressure, and gas used) to form a laminate: theundoped GaN layer 3 with a thickness of 6 μm; the n-typeIn_(X1)Ga_(1-X1)N layer (X1=0.09) 70 with a thickness of 0.1 nm; and thep-type In_(X1)Ga_(1-X1)N layer (X1=0.09) 71 that is doped with Mg andhas a thickness of 0.2 μm. The carrier concentration of the n-typeIn_(X1)Ga_(1-X1)N layer 70 was about 3×10¹⁸ atoms/cm³, and the carrierconcentration of the p-type In_(X1)Ga_(1-X1)N layer 71 was about 3×10¹⁸atoms/cm³. A solar cell element substrate was manufactured by the samemethod as that in Example 1 using the sputter and the laminate.

The solar cell element substrate manufactured in this way was dividedinto square substrates with a length of 1 cm, and the electrodes of thesquare substrate were connected to a lead frame by gold lines, therebymanufacturing a solar cell element.

Example 4

In Example 4, first, as a buffer layer, a single crystal layer made ofAlN was formed on a c-plane of a substrate made of sapphire by an RFsputtering method, and as an underlying layer, a layer made of GaN (agroup-III nitride compound semiconductor) was formed as follows by anMOCVD method. Then, the layers were laminated on the underlying layer.

First, a (0001) c-plane sapphire substrate with a diameter of 2 inches,which was subjected to mirror polishing, was introduced into thechamber. In this case, an RF sputter was used, and an Al target wasused.

Then, the substrate in the chamber was heated up to 500° C., and anitrogen gas was introduced into the chamber. Then, a high-frequencybias was applied to the substrate to generate nitrogen plasma, therebycleaning the surface of the substrate.

Then, while the temperature of the substrate is maintained, argon andnitrogen gases were introduced into the sputter. Subsequently, ahigh-frequency bias was applied to the Al target, and while the internalpressure of the furnace is maintained at 0.5 Pa, a predetermined amountof Ar gas and a predetermined amount of nitrogen gas flowed, therebyforming a single crystal buffer layer made of AlN on the sapphiresubstrate.

Then, the deposition process was performed for a predetermined timeaccording to a predetermined deposition rate to form an AlN (bufferlayer) layer with a thickness of 40 nm, and the plasma process stoppedto reduce the temperature of the substrate.

Subsequently, an X-ray rocking curve (XRC) of the buffer layer formed onthe substrate was measured by an X-ray diffractometer (X'pert Pro MRDmanufactured by SPECTRIS PLC). The result showed that the half-width ofthe XRC of the buffer layer was 0.1°, which is a good value, and thealignment of the buffer layer was good. This measurement was made byusing a CuKα X-ray source as a light source.

Then, the substrate having the AlN layer formed thereon was taken outfrom the sputter, and then carried into an MOCVD apparatus. Then, theunderlying layer made of GaN was formed on the buffer layer as follows.

First, the substrate was put into a reaction furnace (the MOCVDapparatus). Then, a nitrogen gas was introduced into the reactionfurnace, and a heater was operated to increase the temperature of thesubstrate from a room temperature up to 500° C. Then, while thetemperature of the substrate is maintained at 500° C., NH₃ gas andnitrogen gas were introduced to keep the internal pressure of the vapordeposition reaction furnace at 95 kPa. Successively, the temperature ofthe substrate was increased up to 1000° C. to thermally clean thesurface of the substrate. The nitrogen gas was continuously supplied tothe vapor deposition reaction furnace even after the thermal cleaningwas completed.

Thereafter, while continuously supplying ammonia gas, the temperature ofthe substrate was increased up to 1100° C. in a hydrogen atmosphere, andthe internal pressure of the reaction furnace was kept at 40 kPa. Afterchecking that the temperature of the substrate was stabilized at 1100°C., the supply of trimethylgallium (TMG) into the vapor depositionreaction furnace was started, and a process of forming a group-IIInitride compound semiconductor (GaN) forming the underlying layer on thebuffer layer was started. After depositing GaN in this way, a valve ofthe TMG supply pipe was switched to stop the supply of a raw materialinto the reaction furnace, thereby stopping the deposition of GaN.

In this way, the undoped GaN underlying layer with a thickness of 8 μmwas formed on the single crystal AlN buffer layer formed on thesubstrate.

Thereafter, the following were sequentially formed on the underlyinglayer by the same method as that in Example 1 to manufacture a laminatestructure: the n-type GaN layer 50 with a thickness of about 0.5 μm thatwas formed by alternately laminating Si-doped layers (each having athickness of 2 nm), each of which was composed of crystal islands, andundoped GaN layers (each having a thickness 100 nm) as repeated forforty times and by performing a heat treatment on the laminate; and thep-type GaN layer 51 with a thickness of about 0.5 μm that was doped withMg and manufactured by the same method as described above. In this way,a solar cell element was manufactured. In addition, the ohmic electrodelayer made of ITO according to Example 1 was substituted for anelectrode layer made of IZO (bixbyite crystal).

Example 5

In Example 5, a solar cell element was manufactured by the same processas that in Example 3 except that X1 was 0.06 in the n-typeIn_(X1)Ga_(1-X1)N layer (X1=0.09) and the p-type In_(X1)Ga_(1-X1)N layer(X1=0.09).

Example 6

The solar cell element substrate manufactured by the method according toExamples 1 to 4 was divided into square substrates each having a lengthof 1 cm, and the electrodes of the substrate were connected to a leadframe by gold lines, thereby manufacturing a solar cell element. Thesolar cell element having a size of 1 cm² connected to the lead framewas fixed to a sample stage, and probes contacted positive and negativeelectrodes of the lead frame, thereby forming a circuit for measuringcurrent and voltage. Pseudo sunlight (Solar Simulator manufactured byYAMASHITA DENSO CORPORATION) with an energy density of 100 mW/cm² (1SUN) and an AM1.5 spectrum was irradiated, and output characteristicswere measured at the atmosphere temperature and the temperature of thesolar cell below 25±1° C. The results were shown in Table 1.

TABLE 1 Example Open circuit density No. Junction structure voltage[V][mA/cm²] efficiency 1 n-GaN/p-GaN 2.7 1.2 2.4 2 n-GaN/p-GaN 2.6 1.1 2.03 n-type In_(x1)Ga_(1-x1)iN 2.0 1.8 2.5 (X1 = 0.09)/ p-typeIn_(x1)Ga_(1-x1)N (X1 = 0.09) 4 n-GaN/p-GaN 2.8 1.3 2.6 5 n-typeIn_(x1)Ga_(1-x1)N 2.1 1.4 2.1 (X1 = 0.07)/ p-type In_(x1)Ga_(1-x1)N (X1= 0.07)

The use of the manufacturing method and the solar cell element accordingto the invention make it possible to increase the area of the solar cellsubstrate and to considerably reduce manufacturing costs. In addition,it is possible to provide a large solar cell for generating energy bycombining the solar cell according to the invention with a concentrator.

In the present invention, the solar cell includes the buffer layer thatis composed of a group-III nitride semiconductor and formed on thesubstrate, and the group-III nitride semiconductor layers (p-typelayer/n-type layer) having a p-n junction therein. In the solar cell, atleast one selected from the group consisting of the buffer layer and thegroup-III nitride semiconductor layer having the p-n junction thereinhas a compound semiconductor layer formed by a sputtering method. Theuse of the solar cell makes it possible to increase the area of thesubstrate and to considerably reduce manufacturing costs, as compared tothe related art that manufactures a solar cell using only the MOCVDmethod.

Further, according to the manufacturing method according to the presentinvention, the sputtering method is easily combined with other methods(for example, a MOCVD method, an MBE method, a CBE method, and an MLEmethod), which makes it possible to increase the processing speed of asolar cell in each manufacturing process.

In the method of manufacturing a solar cell according to the presentinvention, the use of the sputtering method makes it possible to easilyform a layer by supplying nitrogen in a plasma, radical, or atomic stateand reacting nitrogen with a group-III element. In addition, themanufacturing method includes: a first step in which a process ofsupplying only dopant element and a process of simultaneously supplyinga compound containing a group-III element and a nitrogen raw materialare alternately repeated; and a second step of performing heat treatmenton the layer formed by the first step. Therefore, it is possible to forma high-quality group-III nitride semiconductor layers (p-typelayer/n-type layer).

Furthermore, in the solar cell according to the present invention inwhich the buffer layer that is formed on the substrate and is composedof a group-III nitride semiconductor or the underlying layer formed onthe buffer layer has a compound semiconductor layer formed by thesputtering method, it is possible to reduce threading dislocation byoptimizing the forming conditions of the buffer layer or the underlyinglayer. In particular, the buffer layer formed by the sputtering methodhas a thickness smaller than that of the buffer layer formed by theMOCVD method, but has in-plane uniformity higher than that of the bufferlayer formed by the MOCVD method. Therefore, the buffer layer issingle-crystallized from a polycrystalline material (for example,columnar crystal). For example, it is possible to obtain the underlyinglayer having the following characteristics: the rocking curve half-widthof a (0002) plane is equal to or less than about 100 arcsec; and therocking curve half-width of a (10-10) plane is equal to or less thanabout 300 arcsec. As a result, it is possible to improve the conversionefficiency of a solar cell having a p-n junction that is formed on thebuffer layer.

While preferred embodiments of the invention have been described andillustrated above, it should be understood that these are exemplary ofthe present invention and are not to be considered as limiting.Additions, omissions, substitutions, and other modifications can be madewithout departing from the spirit or scope of the present invention.Accordingly, the invention is not to be considered as being limited bythe foregoing description, and is only limited by the scope of theappended claims.

1. A method of manufacturing a solar cell, comprising: forming a bufferlayer comprising a group-III nitride semiconductor on a substrate usinga sputtering method; and forming a group-III nitride semiconductor layerand electrodes on the buffer layer, wherein the group-III nitridesemiconductor layer is formed on the buffer layer by at least oneselected from the group consisting of the sputtering method, a MOCVDmethod, an MBE method, a CBE method, and an MLE method, and theelectrodes are formed on the group-III nitride semiconductor layer. 2.The method of manufacturing a solar cell according to claim 1, whereinat least one of group-III nitride semiconductor layers (p-typelayer/n-type layer) having a p-n junction therein that are included inthe group-III nitride semiconductor layers formed on the buffer isformed by the sputtering method.
 3. The method of manufacturing a solarcell according to claim 1, wherein the sputtering method comprisesreacting nitrogen in a plasma, radical, or atomic state with a group-IIIelement, thereby forming the layer.
 4. The method of manufacturing asolar cell according to claim 1, further comprising: a first step ofalternately repeating a process of supplying only dopant element and aprocess of simultaneously supplying a compound comprising group-IIIelement and a nitrogen raw material.
 5. The method of manufacturing asolar cell according to claim 4, further comprising: a second step ofperforming a heat treatment on the layer formed by the first step. 6.The method of manufacturing a solar cell according to claim 5, wherein aheat treatment temperature is in a range of about 300° C. to about 1200°C.
 7. The method of manufacturing a solar cell according to claim 5,wherein the heat treatment is performed in an atmosphere that does notcontain a gas of a compound comprising hydrogen atom or hydrogen gas. 8.The method of manufacturing a solar cell according to claim 4, whereinthe dopant element is at least one selected from the group consisting ofSi, Ge, and Sn.
 9. The method of manufacturing a solar cell according toclaim 4, wherein the dopant element is at least one of Mg and Zn.